Vehicle drive system including drive mode-shifting mechanism for shifting between two-wheel drive and four-wheel drive

ABSTRACT

A vehicle drive system includes a compact drive mode-shifting mechanism that is mounted in the final drive gearset of a vehicle. The drive mode-shifting mechanism includes a first shaft, a second shaft, and a clutch assembly. The first shaft forms an input shaft for the differential for a pair of wheels, the second shaft transmits engine power and the clutch assembly connects and disconnects the first and second shafts to one another to effect either the two-wheel-drive mode or the four-wheel-drive mode. The first shaft has an outer surface provided with a first spline, the second shaft has an outer surface provided with a second spline, and said clutch assembly includes an axially movable shifting member provided with an internal spline. The shifting member is movable between a first position in which the internal spline on the shifting member is in engagement with the first spline on the first shaft and the second spline on the second shaft to achieve the four-wheel-drive more and a second position in which the internal spline on the shifting member is in engagement with the first spline on the first shaft and is out of engagement with the second spline on the second shaft to achieve the two-wheel-drive mode.

FIELD OF THE INVENTION

The present invention generally relates to a drive train shiftingmechanism. More particularly, the present invention pertains to avehicle drive system provided with a mechanism for shifting the drivetrain through which engine power is transmitted between two-wheel driveand four-wheel drive.

BACKGROUND OF THE INVENTION

One known mechanism for shifting the drive wheels to which engine poweris transmitted from front or rear wheel drive to all four wheel drive isa transfer case. Generally speaking, the transfer case is fabricatedintegrally with the transmission. The transfer case is specificallyadapted for use a connection with a particular transmission and the useof a different transfer case requires the use of or replacement of manydiffered parts. Thus, the transfer case is not well suited for use withdifferent vehicles mod different transmissions.

Another known arrangement for effecting a shift between two-wheel driveand four-wheel drive is disclosed in Japanese Patent Publication No.23330/1989. Here, a clutch is mounted to one output shaft of thedifferential on the axle side for coupling and uncoupling the enginepower from a part-time four-wheel drive vehicle. When the engine poweris coupled, the clutch is engaged to obtain the four-wheel-drive mode.When the engine power is uncoupled the clutch is disengaged to achievethe two-wheel-drive mode. In this way, the drive mode can be switchedbetween the two-wheel-drive mode and the four-wheel-drive mode. Also, inthe two-wheel-drive mode, the parts of the drive mechanism necessary forthe four-wheel-drive mode are not needlessly driven.

In the arrangement disclosed in the above-cited Japanese PatentPublication No. 23330/1989, the clutch assembly in mounted an the outputshaft side of the differential which undesirably increases the width ofthe differential. This clutch assembly also transmits torque after thefinal gear reduction. In the four-wheel-drive mode, large torque isimposed on the clutch assembly. Therefore, there arises the problem thatthe clutch assembly must be made of a material having great strength atmust be made rather large in size to obtain sufficient strength.

In light of the foregoing, a need exists for a drive-mode shiftingmechanism that does not suffer from the foregoing disadvantages anddrawbacks.

It would thus be desirable to provide a drive-mode shifting mechanismfor switching between two-wheel drive and four-wheel drive that need notbe made of excessively strong material and that is not excessively largein size.

It would also be desirable to provide a drive-mode shifting mechanismfor switching between two-wheel drive and four-wheel drive that has awide range of use and is not limited to use in connection with aparticular vehicle or transmission.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, a vehicle drive systemswitchable between the two-wheel-drive mode and the four-wheel-drivemode includes a transmission, and a housing in which is disposed adifferential gear connected to a pair at wheels and adrive-mode-shifting mechanism. The drive-mode-shifting mechanismincludes a first shaft forming an input shaft for the differential gear,a second shaft for transmitting power from the transmission, and aclutch assembly for connecting the first and second shafts during thefour-wheel-drive mode and for disconnecting and second shaft during thetwo-wheel-drive mode.

In the present invention the clutch assembly is mounted in the finaldrive gearset and so the mechanism for shifting between thetwo-wheel-drive mode and four-wheel-drive mode can be constructed rathersimply by replacing one unit (final drive gearset) including thedifferential. Because the clutch assembly is mounted on the input sideof the differential, the clutch assembly beings the first shaft into orout of engagement with the second shaft before speed is reduced by thegears of the differential. That is, the clutch assembly according to thepresent invention does not require the same degree of high strength asassemblies that are mounted an the output side of the differential.Consequently, where the same material in used a smaller clutch assemblycan be built.

The first and second shafts are preferably mounted coaxially with oneanother, with the first shaft having a first spline on its outer surfaceand the second shaft having a second spline on its outer surface. Theclutch assembly includes a shifting member having an internal splinethat corresponds to the splines on the first and second shafts. Theshifting member is able to move axially to couple the first and secondshafts together by aligning the internal spline of the shifting memberto the splines on both the first and second shafts.

The clutch assembly is advantageously mounted between the bearing thatsupports the first shaft and the bearing that supports the second shaft.In this way advantageous use is made of space which would otherwise bedead space. This permits the size of the assembly to be reduced.Furthermore, this clutch assembly can be used with differenttransmissions and different vehicles with greater ease.

The differential referenced herein refers to a differential assembly forabsorbing the difference in rotational speed between the right and leftdrive wheels or between front and rear drive wheels to permit smoothdriving. The differential can be a limited slip differential. The finaldrive gearset referenced herein refers to the land drive/differentialassembly.

According to another aspect of the present invention, a vehicle drivesystem for a vehicle having four wheels is switchable between atwo-wheel-drive mode in which only two of the wheels are driven and afour-wheel-drive mode in which all four of the wheels are driven, Thevehicle drive system includes a transmission, a differential gear, and adrive mode-shifting mechanism positioned between the transmission andthe differential gear for shifting between the two-wheel-drive mode andthe four-wheel-drive mode. The drive mode-shifting mechanism is housedin a housing that is separate from and spaced from the transmission. Thedrive mode-shifting mechanism includes a first shaft, a second shaft anda clutch assembly for connecting the first shaft and the second shaftduring the four-wheel-drive mode and for disconnecting the first shaftand the second shaft during the two-wheel-drive mode.

Another aspect of the present invention involves a vehicle drive systemswitchable between a two-wheel-drive mode and a four-wheel-drive mode,wherein the vehicle drive system includes a differential gearconnectable to a pair of wheels, and a drive mode-shifting mechanism forshifting between the two-wheel-drive mode and the four-wheel-drive mode.The drive mode-shifting mechanism includes a first shaft forming aninput shaft of the differential gear, a second shaft for transmittingpower and a clutch assembly for connecting the first shaft and thesecond shaft during the four-wheel-drive mode and for disconnecting thefirst shaft and the second shaft during the two-wheel-drive mode.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

The foregoing and additional features and characteristics of the presentinvention will become more apparent from the following detaileddescription considered with reference to the accompanying drawingfigures in which like reference numerals designate like elements andwherein:

FIG. 1 in a schematic diagram of a portion of a vehicle having a vehicledrive system provided with a drive mode-shifting mechanism in accordancewith the present invention;

FIG. 2 is a cross-sectional view of main portions of the drivemode-shifting mechanism shown in FIG. 1; and

FIG. 3 is an enlarged cross-sectional view of a portion of the drivemode-shifting mechanism taken along the section line III—III in FIG. 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A vehicle having a vehicle drive system equipped with a drivemode-shifting mechanism in accordance with the present invention isschematically shown in FIG. 1. In the embodiment discussed below, thevehicle is a rear wheel drive vehicle in which engine power istransmitted to the rear wheels at all times. In the two-wheel-drivemode, power transmission to the front wheels is interrupted.

As illustrated in FIG. 1, the vehicle includes an engine 1 acting as apower source, a transmission 2 connected to the output of the engine 1,a drive axle 4 having one end connected to the output side of thetransmission 2 for transmitting power from the engine 1, and a shaft 17(see FIG. 2) having one end connecting to one end of the drive axle 4 byway of a yoke 16 and the other end connected to a final drive gearset 5.A pair of front axles 6, 8 in connected to the final drive gearset 5,and a front wheel 7, 9 is attached to each of the front axles 6, 8respectively. The vehicle also includes a pair of rear wheels 13, 15,each of which is connected to a respective rear axle 12, 14. The rearaxles 12, 14 are connected to a final drive gearset 11 which is alsoconnected to the transmission 2 by way of a shaft 30 and a yoke 32.

As seen in FIG. 2, a first shaft 18 in located in the final drivegearset 5. This first shaft 18 is mounted coaxially with respect to theshaft 17 that is connected to the drive axle 4 by way of the yoke 16 andto the first drive gearset 5. The shaft 17 constitutes a second shaft.The first shaft 18 is an input shaft to the front differential 20.

The drive mode-shifting mechanism of the present invention is mounted inthe final drive gearset 5 and comprises the first shaft 18, the secondshaft 17 mounted coaxially with the first shaft 18, and a clutchassembly capable of connecting and disconnecting the first shaft 18 andthe second shaft 17. The first shaft is rotatable relative to a housing21. A pinion gear 18b meshing with a ring gear 23 is mounted on the sideof the differential 20, which in turn is mounted inside the housing 21.This differential 20 is housed in a differential case 22 that s mountedso as to be rotatable perpendicular to the axles 6, 8. The ring gear 23meshing with the pinion gear 18b is mounted in the differential case 22.The second shaft 17 is adjacent to the outer end of the first shaft 18and is rotatable relative to the housing 21. The yoke 16 is fixedlymounted to the output shaft of the transmission and to an end of thesecond shaft 17, and transmits engine power to the second shaft 17. Theyoke 16 permits the second shaft 17 to move or pivot relative to thedrive axle 4.

The construction of the clutch assembly is as follows. The clutchassembly includes a shifting member 19 located between the first shaft18 and the second shaft 17, and shifting member 19 is capable ofconnecting and disconnecting the first shaft 18 and the second shaft 17.An internal spline 19a is formed on the inner surface of the shiftingmember 19. This internal spline 19a corresponds to and is capable ofengaging a first spline 18a formed on the outer surface of the firstshaft 18 and a second spline 17a formed on the outer surface of thesecond shaft 17. This shifting member 19 is adapted to slide axiallyalong the splines 18a, 17a on the first and second shafts 18, 17.

A shifting device for producing axial sliding movement of the shiftingmember 19 includes an output gear 25 and a rack 26. As the output gear25 turns, the rack 26 slides relative to a shaft 27 that in mounted tothe housing 21. The output gear 25 in rotated through operation of anelectric motor and the operation of the motor in controlled by anelectronic control unit (ECU). The ECU is connected to, for example, aswitch which affects operation of the motor. The output gear 25 ismounted on the motor via a spiral spring. When the motor is driven,power is transmitted to the output gear 25 via the spring. As the outputgear 25 rotates, the rack 26 moves axially. An element in the form of afork 28 is firmly fixed to the rack 26 and so axial movement of the rack26 also causes axial movement of the fork 28. Accordingly, the shiftingmember 19, which is rigidly affixed to one end of the fork 28, can beaxially slid along the splines on the first and second shafts 18,17through movement of the fork 28.

In accordance with the present invention, a switch-depressing portion28a in mounted integrally with the fork 28. During four-wheel drive, thefork 28 is moved to push the switch-depressing portion 28a against aswitch 31. In this way, the four-wheel-drive mode is electricallydetected. Because the switch 31 in turned on and off, using the fork 28independent of the rotation of the first shaft 18 and the second shaft17, wear of the switch 31 is prevented. As a result, the durability ofthe switch 31 is improved.

The operation of the drive-mode switching mechanism of the presentinvention is as follows. When the vehicle is in the four-wheel-drivemode, the shifting member 19 is splined to both the first shaft 18 andthe second shaft 17. Torque from the output shaft of the transmission 2is transmitted to the yoke 16, the second shaft 17, the first shaft 18,the ring gear 23, and the differential case 22 in this order. Power isthen transmitted via the front axles 6, 8 to the front wheels 7, 9 thatare the drive wheels. When the vehicle makes a turn, the front wheels 7,9 produce a difference in travel with respect to the rear wheels 13, 15,resulting the a difference in rotational speed between the front wheels7, 9 and the rear wheels 13, 15. Therefore, if the four-wheel-drive modeis maintained, the steering effort is undesirably increased.

Accordingly, if the front and rear wheels produce any difference inrotational speed, the output gear 25 is rotated in the oppositedirection from that described above, causing the shifting member 19 toslide axially. This rotational speed differential can be determined byspeed sensors. This disconnects the shifting member 19 from the secondshaft 17. Under this condition, the two-wheel-drive mode is achieved,i.e., engine power is no longer transmitted to the first shaft 18. Inthis way, the difference in rotational speed between the front and rearwheels can be absorbed and converted into rotation between the firstshaft 18 and the second shaft 17. This reduces the steering effort.

However, where the torque between the shifting member 19 and the secondshaft 11 is large and thus the shifting member 19 cannot slide axially,even if the motor operates, the shifting member 19 cannot axially slide.The aforementioned spiral spring mounted between the output shaft of themotor and the output gear 25 then deflects. When the torque in thesecond shaft 17 is reduced to a level that allows the shifting member 19to slide, the output shaft 25 rotates, thus producing sliding movementof the shifting member 19. The shifting member 19 splined to both thefirst shaft 18 and the second shaft 17 is then splined to only the firstshaft 18. As a result only the rear wheels 13, 15 we allowed to bedriven. That is, the two-wheel-drive mode is achieved. The shiftingmember 19 and the fork 28 thus assume the state indicated by the solidline in FIG. 2.

The drive mode is shifted from the two-wheel-drive mode to thefour-wheel-drive mode in the manner described below. To return to thefour-wheel-drive mode, the shifting device actuates the output gear 25so that the shifting member 19 slides axially. The shifting member 19can then engage both the first shaft 18 and the second shaft 17. Thus,torque from the output shaft of the transmission 2 is transmitted to Thefront wheels 7, 9, as well as to the rear wheels 13, 15. If the internalspline 19a on the shifting member 19 and the second spline 17c on thesecond shaft 17 are out of phase and the sliding member 19 cannot slide,the shifting member 19 in unable to slide even if the motor is operated.The aforementioned spiral spring located between the output shaft of themotor and the output gear 25 thus deflects. When the internal spline 19aon the shifting member 19 and the second spline 17a on the second shaft17 are located in phase due to slippage of the rear wheels, for example,the output gear 25 actuates to slide the shifting member 19, thussplining the shifting member 19 to both the first shaft 18 and thesecond shaft 17. This in the instant when a shift is made from thetwo-wheel-drive mode to the four-wheel-drive mode. The shifting member19 and the fork 28 are then in the state indicated by the dot-and-dashhim in FIG. 2.

In, accordance with the present invention, a shift is made at willbetween the two-wheel-drive mode and the four-wheel-drive mode byoperating the clutch assembly. That is the driver operates a shiftingswitch (not shown) to electrically turn on or off the motor or to switchthe direction of electrical energization. Alternatively, rotationalspeed sensors may be mounted on the front and rear wheels. Anydifference in rotational speed between the front and rear wheels is thendetected by the electronic control unit (ECU). The motor is thenelectronically energized to effect a shift from the four-wheel-drivemode to the two-wheel-drive mode. If the ECU senses that the rotationalspeed difference bas decreased to zero, the direction of energization ofthe motor is reversed as compared to the situation where a shift is madeto the two-wheel-drive mode. In this way, a shift is made from thetwo-wheel-drive mode to the four-wheel-drive mode.

According to the present invention, when the splined shafts are out ofphase or the torque is large, the sliding movement of the shiftingmember 19 is limited through use of the spiral spring. It is to beunderstood, however, that the use of the spiral spring is not essentialto the present invention. Rather, any type of mechanism may be used aslong as the shifting member 19 is inhibited from sliding before thesplines are located in phase with one another.

Also, in the present invention, the switching member is mounted betweenthe bearing 29 supporting the first shaft 18 and the bearing 30supporting the second shaft 17. Consequently, use is advantageously madeof space which would have been dead space in other known constructions.This permits the assembly to be made smaller.

Furthermore, this clutch assembly can be used without having to make anymodifications to the transmission or without having to use a differenttransmission. In addition, the steering force can be reduced by making ashift from the four-wheel-drive mode to the two-wheel-drive mode.

According to the present invention, engine power is transmitted directlyto the output axle without being transmitted via a transfer ease. Theinvention can also be applied to a structure in which a transfer case ismounted between the transmission and the output axles of the front andrear wheels. In the embodiment described above, the differential 20 is alimited-slip type differential. However, as an alternative, an open typedifferential can be used.

In accordance with the present invention, the clutch assembly is mountedin the final drive gearset. Therefore, a mechanism permitting the driverto effect a shift between the two-wheel-drive mode and thefour-wheel-drive mode can be constructed simply by replacing only thefinal drive gearset including the differential. Also, the clutch numblyis mounted on the input side of the differential. Consequently, theclutch amounts and disconnects the first And second shafts before thespeed is reduced by the gears of the differential. That is, the clutchassembly according to the present invention does not require highstrength, unlike other known clutch assemblies mounted an the outputside of the differential. Consequently, where the same material is used,a smaller clutch assembly can be built.

As noted above, by mounting the clutch assembly between the two bearingsthat support the first and second shafts, advantageous use is made ofspace which would otherwise have been dead space. This permits the sizeof the assembly to be reduced. Further, the clutch assembly of thepresent invention can advantageously be used without the need forsignificant modifications to the transmission and without the need fordifferent transmissions.

The principles, a preferred embodiment and mode of operation of thepresent invention have been described in the foregoing specification.However, the invention which is intended to be protected is not to beconstrued as limited to the particular embodiment described. Further,the embodiment described herein is to be regarded as illustrative ratherthan restrictive. Variations and changes may be made by others, andequivalents employed, without departing from the spirit of the presentinvention. Accordingly, it is expressly intended that all suchvariations, changes and equivalents which fall within the spirit andscope of the invention be embraced thereby.

1. A vehicle drive system switchable between two-wheel-drive mode and afour-wheel-drive mode comprises: a transmission; a housing in which isdisposed a differential gear connected to a pair of wheels and adrive-mode-shifting mechanism; the drive-mode-shifting mechanismincluding a first shaft forming an input shaft for the differentialgear, a second shaft for transmitting power from the transmission, and aclutch assembly for connecting the first and second shafts during thefour-wheel-drive mode and for disconnecting the first and second shaftsduring the two-wheel-drive mode; and the clutch assembly being operatedby an electric motor.
 2. A vehicle drive system according to claim 1,including a drive axle extending between the transmission and thehousing.
 3. A vehicle drive system according to claim 1, wherein saidfirst and second shafts are mounted coaxially.
 4. A vehicle drive systemaccording to claim 1, wherein said firm shaft has an outer surfaceprovided with a first spline, and second shaft has an outer surfacesprovided with a second spline, and said clutch assembly includes anaxially movable shifting member provided with an internal spline, saidshifting member being movable between a first position in which theinternal spline on the shifting member is in engagement with the firstspline on the first shaft and the second spline on the second shaft toachieve the four-wheel-drive mode and a second position in which theinternal spline on the shifting member is in engagement with the firstspline on the first shaft and is out of engagement with the secondspline on the second shaft to achieve the two-wheel-drive mode.
 5. Avehicle drive system according to claim 4, including a shifting devicefor moving the shifting member between the first and second positions.6. A vehicle drive system according to claim 5, wherein the shiftingdevice includes an element fixed to the shifting member, a rack to whichis fixed the element, and a gear for moving the rack.
 7. A vehicle drivesystem according to claim 6, including a switch-depressing portionprovided on said element, and a switch operated by the switch-depressingportion during movement of said element.
 8. A vehicle drive system for avehicle that includes four wheels and is switchable between atwo-wheel-drive mode in which only two of the wheels are driven and afour-wheel-drive mode in which all four of the wheels are drivencomprising: a transmission; a differential gear; a drive mode-shiftingmechanism positioned between the transmission and the differential gearfor shifting between the two-wheel-drive mode and the four-wheel-drivemode, the drive mode-shifting mechanism including a first shaft, asecond shaft and a clutch assembly for connecting the first shaft andthe second shaft during the four-wheel-drive mode and for disconnectingthe first shaft and the second shaft during the two-wheel-drive mode;the clutch assembly being operated by an electric motor to effect theshifting between the two-wheel-drive mode in which the first shaft andthe second shaft are disconnected and the four-wheel-drive mode in whichthe first shaft and the second shaft are connected; and the vehicledrive system being devoid of a differential between the transmission andthe drive mode-shifting mechanism.
 9. A vehicle drive system accordingto claim 8, including a housing which houses the drive mode-shiftingmechanism and the differential gear.
 10. A vehicle drive systemaccording to claim 8, wherein said first and second shaft are mountedcoaxially.
 11. A vehicle drive system according to claim 8, including adrive axle extending from the transmission, and a yoke connected a thedrive axle and to an end of the second shaft opposite the first shaftfor permitting the drive axle and the second shaft to pivot relative toour another.
 12. A vehicle drive system according to claim 8, whereinsaid first shaft forms an input shaft for the differential gear.
 13. Avehicle drive system according so claim 8, wherein said first shaft hasan outer surface provided with a first spline, said second shaft has anouter surface provided with a second spline, and said clutch assemblyincludes an axially movable shifting member provided with an internalspline, said shifting member being movable between a first position inwhich the internal spline on the shifting member is in engagement withthe first spline on the first shaft and the second spline or the secondshaft to achieve the four-wheel-drive mode and a second position inwhich the internal spline on the shifting member is in engagement withthe first spline on the first shaft and is out of engagement with thesecond spline on the second shaft to achieve the two-wheel-drive mode.14. A vehicle drive system according to claim 13, including a shiftingdevice for moving the shifting member between the first and secondpositions.
 15. A vehicle drive system according to claim 14, wherein theshifting device includes an element fixed to the shifting member, a rackto which is fixed the element, and a gear for moving the rack.
 16. Avehicle drive system according to claim 15, including aswitch-depressing portion provided on said element, and a switchoperated by the switch-depressing portion during movement of saidelement.
 17. A vehicle drive system switchable between a two-wheel-drivemode and a four-wheel-drive mode, comprising: a drive axle; adifferential gear connectable to a pair of wheels; a drive mode-shiftingmechanism for shifting between the two-wheel-drive mode and thefour-wheel-drive mode, the drive mode-shifting mechanism including afirst shaft forming an input shaft of the differential gear, a secondshaft for transmitting power, and a clutch assembly for connecting thefirst shaft and the second shaft during the four-wheel-drive mode andfor disconnecting the first shaft and the second shaft during thetwo-wheel-drive mode; the clutch assembly including a shifting memberaxially shifted through operation of an electric motor to effect theswitching between the four-wheel-drive mode and the two-wheel-drivemode; and a yoke connected to the drive axle and an end of the secondshaft opposite the first shaft for permitting the drive axle and thesecond shaft to pivot relative to one another.
 18. A vehicle drivesystem according to claim 17, wherein said first and second shafts aremounted coaxially.
 19. A vehicle drive system according to claim 17,wherein said first shaft has an outer surface provided with a firstspline, said second shaft has an outer surface provided with a secondspline, and said clutch assembly includes an axially movable shiftingmember being provided with an internal spline, said shifting memberbeing movable between a first position in which the internal spline onthe shifting member is in engagement with the first spline on the firstshaft and the second spline an the second shaft to achieve thefour-wheel-drive mode and a second positions in which the internalspline on the shifting member is in engagement with the first spline anthe first shaft and is out of engagement with the second spline on thesecond shaft to achieve the two-wheel-drive mode.
 20. A vehicle drivesystem according to claim 19, including a shifting device for moving theshifting member between the first and second positions, said shiftingdevice including as element fixed to the shifting member, a rack towhich said element is fixed, and a gear for moving the rack.
 21. Avehicle drive system according to claim 1, including a first splinedirectly formed on an outer surface of the first shaft, the clutchassembly including an axially movable shifting member that is axiallymoved through operation of the electric motor, the shifting memberpossessing an inner surface with an internal spline formed on the innersurface of the shifting member, the internal spline of the shiftingmember being engageable with the first spline of the first shaft.
 22. Avehicle drive system according to claim 1, wherein the clutch assemblyincludes an axially movable shifting member, the electric motor beingoperatively connected to an element which engages the shifting member toaxially move the shifting member during operation of the electric motor.23. A vehicle drive system according to claim 22, wherein the elementengaging the shifting member is a fork.
 24. A vehicle drive systemaccording to claim 1, wherein the four-wheel-drive mode is electricallydetected.
 25. A vehicle drive system according to claim 8, including afirst spline directly formed on an outer surface of the first shaft, theclutch assembly including a shifting member possessing an inner surfacewith an internal spline formed directly on the inner surface of theshifting member, the internal spline of the shifting member beingengageable with the first spline of the first shaft.
 26. A vehicle drivesystem according to claim 25, wherein the internal spline of theshifting member engages the first spline of the first shaft during boththe four-wheel-drive mode and the two-wheel-drive mode.
 27. A vehicledrive system according to claim 25, wherein the electric motor isoperatively connected to an element which engages the shifting member toaxially move the shifting member during operation of the electric motor.28. A vehicle drive system according to claim 8, wherein thefour-wheel-drive mode is electrically detected.
 29. A vehicle drivesystem according to claim 17, including a first spline directly formedon an outer surface of the first shaft, the shifting member possessingan inner surface with an internal spline formed directly on the innersurface of the shifting member, the internal spline of the shiftingmember being engageable with the first spline of the first shaft.
 30. Avehicle drive system according to claim 29, wherein the internal splineof the shifting member engages the first spline on the first shaftduring both the four-wheel-drive mode and the two-wheel-drive mode. 31.A vehicle drive system according to claim 17, wherein the electric motoris operatively connected to an element engaging the shifting member toaxially move the shifting member during operation of the electric motor.32. A vehicle drive system according to claim 17, wherein the fourwheel-drive-mode is electrically detected.
 33. A vehicle drive systemswitchable between two-wheel-drive mode and a four-wheel-drive modecomprises: a transmission; a housing in which is disposed a differentialgear connected to a pair of wheels and a drive-mode-shifting mechanism;the drive-mode-shifting mechanism including a first shaft forming aninput shaft for the differential gear, a second shaft for transmittingfrom the transmission, and a clutch assembly for connecting the firstand second shafts during the four-wheel-drive mode and for disconnectingthe first and second shafts during the two-wheel-drive mode; the firstshaft having an outer surface with a first spline directly formed on theouter surface of the first shaft; the clutch assembly including anaxially movable shifting member having an inner surface with an internalspline formed on the inner surface of the shifting member, the internalspline of the shifting member and the first spline of the first shaftengaging one another in both the two-wheel-drive mode and thefour-wheel-drive mode; the shifting member being engaged by an elementwhich is operatively connected to an electric motor so that operation ofthe electric motor axially moves the shifting member between oneposition in the two-wheel-drive mode in which the first and secondshafts are disconnected and another position in the four-wheel-drivemode in which the first and second shafts are connected to one anotherby the shifting member.
 34. A vehicle drive system according to claim33, wherein the four-wheel-drive mode is electrically detected.